BIOLOGY  
 
CELLS – the building blocks of life

Cells are the structural and functional units of all living organisms. It is the smallest unit and also called the building blocks of life. Some living organisms are UNICELLULAR and some are MULTICELLULAR. The cell theory was first developed by Matthias Jakob Scheleiden and Theodor Schwann in 1839, stating that all living organisms are made up of one or more cells and that all vital functions of organisms occur in these cells. Robert Hooke coined the term ‘Cell’ in 1665. 		 
   

TYPES OF ORGANISMS

  
  The unicelluar oraganisms function on just one cell all their lives. All their essential metabolic functions like resipiration, assimilation, excretion and reproduction; are carried out by the single cell itself.
The mulitcellular organisms divide their single cell and form more cells gradually.



STRUCTURE OF AN ONION PEEL CELL: Cells differ in shape and structure depending on their functions. There is no specific shape for a cell.

 



THE UNICELLULAR ORGANISMS FUNCTION ON JUST ONE CELL ALL THEIR LIVES. ALL THEIR ESSENTIAL METABOLIC FUNCTIONS ; LIKE RESPIRATION, ASSIMILATION, EXCRETION AND REPRODUCTION; ARE CARRIED OUT BY THEIR SINGLE CELL ITSELF.

THE MULTICELLULAR ORGANISMS DIVIDE THEIR SINGLE CELL AND FORM MORE CELLS GRADUALLY.

STRUCTURE OF AN ONION PEEL CELL:

 

 

Cells differ in shape and structure depending on their functions. There is no specific shape for a cell.

 

COMPONENTS OF A PLANT CELL:

 

           		 
 
CELL WALL – a thick, rigid membrane surrounding a plant cell. It is made of cellulose fibre and gives the cell its basic support and shape. It bonds with the other cell walls to form the plant structure.

CELL MEMBRANE – this thin surrounding layer is made of proteins and fats and lies within the cell wall. It is semipermeable i.e. it allows a few substances to pass through while blocking the others.

VACUOLE – it is a large space filled with fluid within a plant cell which helps the cell to maintain its shape.

NUCLEUS – it is spherical in shape and contains the DNA chromosomes alongwith the nucleolus and many other organelles. It controls the protein synthesis thereby controlling many important functions the cell. It is surrounded by a nuclear membrane.

NUCLEOLUS – it is an organelle within the nucleus and produces the RNA (ribonucleic acid).

NUCLEAR MEMBRANE – it is the membrane surrounding the nucleus.

CHLOROPLAST – it is a disc-shaped organelle containing the chlorophyll which helps in Photosynthesis i.e. formation of carbohydrates from carbon dioxide and water using sunlight as an energy source.

MITOCHONDRION – these are spherical to rod shaped organelles with a double membrane. Its inner membrane lies in many folds to make projections called cristae. They convert energy stored in glucose into ATP (adenosine triphosphate).

CYTOPLASM – it is a jelly-like material around the nucleus containing the organelles.

AMYLOPLAST- it is a starch-storing, colorless plastid which occurs only in plant storage tisues.

CENTROSOME – it is a small body containing centrioles. It provides the microtubules and helps in cell-division.

ROUGH ENDOPLASMIC RETICULUM – it is a complicated system of interconnected, membranous, infolded, convoluted sacks that are located in the cell’s cytoplasm. Its rough appearance is due to the ribosomes that cover it. It transfers materials through the cell and produces proteins in sacks called cristernae.

SMOOTH ENDOPLASMIC RETICULUM – it is similar to the rough ER, infact it buds off from it. The space within it is called the ER lumen. It contains enzymes and produces and digests fats and membrane proteins.

RIBOSOMES – these are small organelles composed of RNA-rich cytoplasmic granules and are sites of protein synthesis.

GOLGI BODY – these are flattened, layered, sac-like organelles which are located near the nucleus.

COMPONENTS OF AN ANIMAL CELL:



CELL MEMBRANE - this thin surrounding layer is made up of proteins and fats. It is semipermeable i.e. it allows a few substances to pass through while blocking the others.

LYSOSOME – also known as cell vesicles, they are round organelles surrounded by a membrane and help in the digestion of cell nutrients as they contain the digestive enzymes.

NUCLEUS – it is spherical in shape and contains the DNA chromosomes alongwith the nucleolus and many other organelles. It controls the protein synthesis thereby controlling many important functions the cell. It is surrounded by a nuclear membrane.

NUCLEOLUS – it is an organelle within the nucleus and produces the RNA (ribonucleic acid).

NUCLEAR MEMBRANE – it is the membrane surrounding the nucleus.

VACUOLE – it is a large space filled with fluid within a plant cell which helps the cell to maintain its shape.

MITOCHONDRION – these are spherical to rod shaped organelles with a double membrane. Its inner membrane lies in many folds to make projections called cristae.

CENTROSOME – it is a small body containing centrioles. It provides the microtubules and helps in cell-division.

CYTOPLASM – it is a jelly-like material around the nucleus containing the organelles. ROUGH

ENDOPLASMIC RETICULUM – it is a complicated system of interconnected, membranous, infolded, convoluted sacks that are located in the cell’s cytoplasm. Its rough appearance is due to the ribosomes that cover it. It transfers materials through the cell and produces proteins in sacks called cristernae.

SMOOTH ENDOPLASMIC RETICULUM – it is similar to the rough ER, infact it buds off from it. The space within it is called the ER lumen. It contains enzymes and produces and digests fats and membrane proteins. GOLGI BODY – these are flattened, layered, sac-like organelles which are located near the nucleus.

SOME MORE CELL ORGANELLES: PEROXISOMES – cell organelles containing enzymes that catalyze the production and breakdown of Hydrogen Peroxide.

PLASTIDS – these are organelles found in plants and certain other organisms. Like mitochondria, they have an outer as well as an inner membrane and contain their own DNA and ribosomes. Some plastids contain pigments also (like chloroplast).

CHROMOPLAST – it is a kind of plant cell that synthesizes and contains pigments other than chlorophyll (mostly yellow or orange carotenoids).

LEUCOPLASTS – these are colorless plastids found in the cytoplasm of plant cells and make and store starch.

DIFFERENCE BETWEEN PLANT AND ANIMAL CELLS:

BASIS PLANT CELLS ANIMAL CELLS
CELL WALL Cell wall made of cellulose is present Cellulose in any form including cell wall is absent
CYTOPLASM It is pushed to the periphery and forms a thin line within the cell wall It is denser and occupies the maximum space in the cell
CENTROSOME Centrosome is absent, polar caps are present instead Centrosome is present
VACUOLE On or more large and prominent vacuoles are present mostly absent, if present, they are small and temporary
PLASTID Plastids are present Plastids are absent
SIZE Comparatively larger in size Usually small in size